Keep the 17057504, it is an excellent carburetor for what you are doing. I run a very similiar Pontiac model on my 455 making over 550hp, it is part number 17057274.
The 504 is an California emissions carburetor, so it's going to need some help for the planned engine build. Check the head casting numbers on your 350, hopefully they are 1973 vintage instead of the later "smog" heads. The 73 castings will end in "993", they were, and are excellent cylinder heads. They were the last castings to retain the same ports as the earlier pre-smog castings. They did get a large combustion chamber and smaller spark plug like all the later heads. The later 882 and 624 castings are not nearly as good, being very restrictive in port flow compared to earlier models.
The EGR intake is fine, don't waste any money on an aftermarket aluminum intake. They woln't bring anything to the table for what you are doing. The iron holds the heat better anyhow.
When you are ready to do the carb build, call the shop, we have everything required to get that carburetor up to par for this new fuel, and dial it in for your engine without a lot of guesswork.
Here's a video of my car back in August at the Tri Power Nationals in Norwalk. My plain old 17047274 carburetor took me to the finals once again, it's never had a grinder or sanding roll touch it. It uses one of our electric choke conversions, and is upgraded with the same parts used in our rebuild kits.....Cliff
http://www.youtube.com/watch?v=6zVdoLR-VzM&context=C30b6750ADOEgsToPDskLN6xs9w6YNF-3XYco3MFqf
